The subject of carbon dioxide management is paramount in the overall strategy of greenhouse gas reduction. Carbon dioxide management in this context can include reduction, containment and conversion, as well as combinations of these approaches. While reduction of new carbon dioxide emissions is critical in any future anti-climate-change environmental strategy, it does not address the enormous inventory of present carbon dioxide in the ecosystem, nor does it address the current momentum of generating new emissions. For that reason, there is an important emphasis on developing technologies that can efficiently capture carbon dioxide (preferably at point discharge sites) and use it as part of a regeneration cycle for new fuel sources.
The use of semipermeable membranes for effecting gas separations has become well accepted, and membranes of various polymeric and inorganic configurations display various degrees of separation, across a broad spectrum of gases and gas mixtures. Such semipermeable membranes are available in flat sheet, tubular, spiral wound and hollow fiber configurations, and many membranes exhibit good separation factors, i.e. 2.5-50 to 1 for carbon dioxide and methane, and 2.5-100 to 1 for CO2 and nitrogen, as well as high permeabilities at fairly low net driving pressures. If these strategies are to be successful, it may often be required to be able to accept a variation of feed inputs and then process them so as to control and regulate the gaseous mixtures to obtain a desired mixture of carbon dioxide, methane and/or nitrogen for supply to a subsequent process.
As mentioned above, carbon dioxide capture is only part of an effective carbon dioxide management strategy; an important part is providing an efficient means of converting the carbon dioxide into high value fuel (and non-fuel) products, as such will alleviate the need to bring new carbon into the overall fuel cycle. Generally, carbon dioxide conversion has proven to be an energy-intensive process, and such can negate an overall objective of energy efficiency. Plasma technology, however, has emerged as one approach for the efficient conversion of carbon dioxide, particularly in gas mixtures where hydrogen source gases are present. Microwave plasmas are particularly efficient in these processes, with reported energy costs as low as 0.15 kWh per cubic meter of hydrogen gas produced from the reformation of methane and carbon dioxide.
Dickman et al U.S. Pat. No. 7,682,718 discloses a fuel management system for a hydrogen fuel cell; the system comprises a number of tanks that can be controllably filled and mixed from a variety of feeds as part of the required fuel mix for the fuel cell. Adamopoulos et al U.S. Pat. No. 7,637,984 uses an adsorbent material to first remove sulfur from a gas stream which is then treated with a membrane system to separate carbon dioxide from a hydrogen-rich stream.
Wei et al U.S. Pat. No. 7,648,566 mentions the use of inorganic and polyether membrane systems for the purpose of separating carbon dioxide from a Syngas stream in order to produce an enriched hydrogen stream as a part of a pre-combustion carbon dioxide capture process. Muradov et al U.S. Pat. No. 7,588,746 mentions possibly using a membrane system, a pressure swing adsorption system or a cryogenic adsorption unit for treating combustion gas from hydrogen combustion to separate hydrogen from other gases (including methane).
Hoffman et al U.S. Pat. No. 7,634,915 suggests that zeolite and ceramic membranes may be used to separate a carbon dioxide rich stream from a carbon dioxide lean stream (where the carbon dioxide lean stream may contain carbon monoxide, nitrogen and unspent fuel such as methane) as a part of a turbine system for producing hydrogen and isolating carbon dioxide. In this system, the carbon dioxide is used for combustion temperature regulation and turbine cooling. Hemmings et al U.S. Pat. No. 7,686,856 discloses a system for Syngas production using water and methane reforming; in this system, an oxygen transport membrane is used as part of the combustion process to produce the Syngas products. Murphy U.S. Pat. No. 5,277,773 discloses a microwave plasma used for a reformer reaction including water and a hydrocarbon where the plasma reaction is initiated using one or more metallic wire segments.
It is known that microwave plasma technology can be used to reform gas streams which contain specific concentrations of CO2 and CH4 with a mole ratio of not greater than about 1.5 to 1, i.e. carbon dioxide (in the range of 40-60 mole percent) and methane (in the range of 60-40 mole percent), into a carbon monoxide and hydrogen (Syngas) mixture (see U.S. Pat. Nos. 4,975,164 and 5,266,175). Such a product can be used as a feedstock for a conventional Fischer-Tropsch (F-T) synthesis (see U.S. Pat. Nos. 6,596,780 and 6,976,362, the disclosures of which are incorporated herein by reference) that will convert such a gaseous mixture to liquid hydrocarbons. However, it is most important that efficiencies in operation be found before such strategies can become an economic reality.
Notwithstanding the advancement in both the areas of membrane technology and plasma technology, there is a present need for the integration of these technologies in a manner which renders such membrane separations able to function as useful, “tunable” elements in an integrated gas management system and plasma reformer process. In particular, there is a need for a membrane-based gas control system that is capable of providing an optimum gas mixture feedstream, created from a variety of carbon dioxide sources, to a microwave plasma reformer.
Accordingly, it is one of the objectives of the present invention to provide a membrane-based system that can capture carbon dioxide directly from a variety of sources, concentrate it, combine such concentrate with methane gas, and controllably and economically create an optimum gas mixture for use as a feedstream to a microwave plasma reformer for the production of Syngas.
It is another objective of the present invention to provide a process and system that, in addition to the above, can co-generate sufficient electrical energy to power such a microwave plasma reformer.
It is yet another objective of the present invention to provide a process as set forth above integrated in combination with a Fischer-Tropsch liquid hydrocarbon production process.